Non-stick coating system with two thin undercoats, the first being polysiloxane

ABSTRACT

Non-stick coating systems can be applied to untreated smooth glass and ceramic substrates with a polysiloxane undercoat overlaid with a second undercoat of polyamide imide preferably containing colloidal silica and surfactant, with each undercoat in the range of 0.1-5.0 microns thick.

BACKGROUND OF THE INVENTION

This invention relates to coatings systems, such as for cookware, whichminimize sticking. More particularly, it relates to multilayer coatingsystems that can be used on smooth, untreated substrates.

Generally in the art a metal or glass substrate is roughened by somemeans before the first layer of coating is applied so that mechanicalbonding will assist chemical adhesive means in holding the coating ontothe substrate. Typical roughening means include acid etching,sand-blasting, grit-blasting, and baking a rough layer of glass, ceramicor enamel frit onto the substrate. The problem of adhesion of non-stickcoatings to substrates is exacerbated by the nature of the coatings. Ifthe coating is optimized for release to prevent food particles fromsticking to it, easy clean-up after cooking, durability or to facilitatelow friction sliding contact, almost by definition there will bedifficulties in making it adhere well to the substrate.

The substrate can be metal, often aluminum or stainless steel used forcookware or industrial applications. It can be glass or ceramic. Itmight even be plastic for microwave oven cookware, or it could be anindustrial article such as a saw made of carbon steel. Whatever thesubstrate or the application, if it is necessary to roughen thesubstrate to make the coating adhere, that at least adds cost and cancause other difficulties including creating a rough profile which canprotrude or telegraph through the coating. This is especiallyundesirable when smoothness is sought, such as for saws and steam irons.The environmental cost of disposing of etchant materials can besignificant. Sometimes, especially for glass and ceramic substrates, italso can cause unacceptable weakness or brittleness of the substrate.

Means of enhancing adhesion of non-stick coatings to a substrate areillustrated by the following patents.

U.S. Pat. No. 2,562,118-Osdal (1951) uses a mixture of phosphoric andchromic acids in a coating containing polytetrafluoroethylene (PTFE) foruse directly on metal or glass. This can cause some inherent etching ofthe substrate. Better adhesion would be desirable.

U.S. Pat. No. 4,049,863 - Vassiliou (1977) teaches a primer containingfluoropolymer, such as PTFE, colloidal silica and a polyamide imide(PAI), along with other constituents, applied by various techniques to asubstrate that is preferably pretreated by grit blasting, flame sprayingof metals or metal oxides or frit coating, or to phosphated andchromated metals. The PTFE:PAI ratio can be 1:9. The primer coat isordinarily applied to a dry film thickness (DFT) of about 2-15 microns(μm). After air drying, the primer is topcoated with a conventionalfluoropolymer enamel and baked. (Parts, percentages and proportionsherein are by weight except where indicated otherwise.)

Although U.S. Pat. No. 4,259,375 - Vassiliou (1981) teaches applying adecorative fluoropolymer coating on a primer which is applied directlyto the substrate, the implication is that the usual roughened substrateis used.

U.S. Pat. No. 3,801,379 - Blackwell (1974) applies a poly(arylenesulfide) coating such as polyphenylene sulfide (PPS) directly to analuminum substrate after treating the substrate with hot water or steam.Preferably the coating is done with the substrate at 455° C., causingflocking of the coating. The thickness of the coating is not disclosed,and the PPS can contain 5-20% PTFE.

Canadian 887,122 - Fang (1971) provides a single coating of PAI and PTFEon a metal substrate with a concentration gradient from mostly PAI atthe substrate to mostly PTFE at the air interface. This is applied as asingle coat without any special primer on ordinary steel or sand-blastedaluminum.

British 1,512,495 (SEB) (1978) applies a PAI coating containing PTFEpowder at a DFT of 5-100 microns to a metal or glass substrate that hasbeen merely de-greased and not roughened. The minimum ratio of PTFE:PAIis 1:7. This then is the undercoat on which a PTFE powder topcoat isapplied. However, the PTFE in the undercoat at these thicknesses canlead to failure of intercoat adhesion.

European application 100,889 - Gebauer (1984) describes thicker coatingscontaining certain perfluorocarbon resins, PPS, lithium hydroxide, andsilica and asserts they are superior to prior art primer coatings on theorder of 10 microns thickness. If the substrate is smooth, he teachesthat it can be roughened by sand blasting or etching.

U.S. Pat. No. 4,177,320 and U.S. Pat. No. 4,139,576 - both Yoshimura, etal, (1979) teach an undercoat of PPS or PPS plus PAI or polyimide withsurface active agents on sand-blasted rough aluminum at a DFT of 15 or20-25 microns. As in many of these patents, the need for roughening thesubstrate to obtain adequate adhesion and durability adds considerablecosts.

Coatings systems including primers, intermediate and top coats usefulwith the present invention are described in various patents including:

U.S. Pat. No. 4,049,863 - Vassiliou (1977);

U.S. Pat. No. 4,118,537 - Vary and Vassiliou (1978);

U.S. Pat. No. 4,123,401 - Berghmans and Vary (1978);

U.S. Pat. No. 4,252,859 - Concannon and Vary (1981);

U.S. Pat. No. 4,351,882 -Concannon (1982); all incorporated herein byreference.

SUMMARY OF THE INVENTION

The present invention, in certain of its embodiments, provides a coatingsystem comprising a substrate, preferably of glass or ceramic, with amulti-layer non-stick coating, comprising two undercoats, a primer, atopcoat and up to one or more intermediate coats, wherein:

the substrate is free of contaminants that would prevent adhesion of theundercoat,

the first undercoat is applied to the substrate and consists essentiallyof polysiloxane resin and has a thickness in the range of 0.1-5.0microns,

the second undercoat is applied to the first undercoat and consistsessentially (on a weight basis after air drying but before baking) up to8%, preferably 3-8%, surfactant, up to 20%, preferably 10-20%,fine-particle silica, the balance polyamide imide, and has a dry filmthickness in the range of 0.1-5.0 microns,

the primer is applied to the second undercoat and comprisesperfluorocarbon resin and at least one of polyamide imide, polyarylenesulfide and polyether sulfone and has a dry film thickness in the rangeof 2-15 microns, and

the topcoat comprises perfluorocarbon resin.

DETAILED DESCRIPTION

The present invention permits not only lower cost by avoiding theroughening of the substrate but also smoother coated surfaces which canbe advantageous for release on cookware.

When coating systems otherwise according to the invention are attemptedon smooth substrates without the required thin undercoat, the film canbe removed too easily by ordinary crosshatch or finger-nail scratchtests. Also, when the undercoat is thicker than stated above, it tendsto crack the midcoat and cause rippling of the topcoat. With use of theundercoat on smooth substrates, treated only by washing to remove greaseand any other contaminants which might interfere with adhesion, coatingsystems of the invention give good food release and good resistance tousual durability tests such as the "tiger paw" cooking tests involving aweighted holder with multiple ball point pen shafts rotating around theinside of a frying pan during cooking tests. Satisfactory results arenot obtained without the undercoat or with PTFE in the undercoat or withtoo thick an undercoat.

Typical prior art preparation of surfaces to enhance adhesion of arelease coating has involved etching or sand or grit blasting to developa surface profile. The profile measured in average microinches using amodel RT 60 surface roughness tester made by Alpa Co. of Milan, Italy.The present invention is particularly useful with substrates having aprofile of less than 100, preferably less than 50 micro inches.

In the following examples, the polysiloxane resin is that of U.S. Pat.Nos. 4,369,279 - Emerich (1983), 4,252,702 - Wald (1987), and4,183,998 - Vassiliou (1980); the polyamide imide, colloidal silica anddispersions preferably are those of U.S. Pat. No. 4,049,863 - Vassiliou(1977); the PTFE blends are those of U.S. Pat. Nos. 4,252,859 and4,351,883 - Concannon (1982); the ultramarine blue is that of U.S. Pat.No. 4,425,448 - Concannon and Rummel (1984); the polyether sulfone isthat of U.S. Pat. Nos. 3,981,945 (1976), 4,090,993 (1978) - both Atwoodet al, and U.S. Pat. No. 4,131,711 (1978) - Atwood, and the polyarylenesulfide is the polyphenylene sulfide of U.S. Pat. No. 4,287,112 (1981) -Berghmans.

EXAMPLE

A piece of "Pyrex" borosilicate glass made by Corning Glass is washed toremove contaminants. It is then coated with a thin layer of polysiloxaneresin formulation to a DFT of 0.5-5.0, preferably 0.25-2.5, morepreferably 1.25 microns and air dried. The formulation is:

    ______________________________________                                        FIRST UNDERCOAT                                                               Ingredient         % by weight                                                ______________________________________                                        polysiloxane resin 21.0                                                       6-2230 from Dow Corning                                                       petroleum xylol    39.5                                                       toluene            39.5                                                       TOTAL              100.00                                                     ______________________________________                                    

This produces a first undercoat on which is then applied a secondundercoat as follows.

The second undercoat comprising polyamide imide, colloidal silica andsurfactant is sprayed on the first undercoat to a DFT of 0.1-5.0,preferably 0.25-2.5, more preferably 1.25 microns. This undercoat ispermitted to air dry, then a primer coat of polyamide imide, PTFE,colloidal silica, TiO₂ -coated mica, pigmentation such as ultramarineblue, TiO₂ and other pigments, is sprayed onto the undercoat, to a DFTof 5.0 to 7.5 microns, and a topcoat is sprayed to a DFT of 15 to 17.5microns (wet-on-wet) on the primer coat, containing PTFE, decomposableacrylic resin to aid in coalescence, and Tio₂ coated mica. The coatingsystem is then baked at 149° C. for 10 minutes followed by 427° C. for 5minutes. The substrate should be above 416° C. at least 3 minutes in acommercial operation where the thermal inertia of the substrate cancause slow heating. As is shown in the art, DFT can be controlled by thetime of spraying or by dilution, and other application techniques suchas roller coating can be used.

The thicknesses of the undercoats and the primer coat are both importantto avoid loss of intercoat adhesion. The topcoat thickness is preferredbut can be varied. Thicker topcoats under some circumstances can giveimproved performance. The primer coat weight ration of PTFE to PAIpreferably is in the range of 2:1 to 7:1 with about 4:1 most preferred.

The following tables show the formulations for these layers.

    ______________________________________                                        SECOND UNDERCOAT                                                                             % by weight                                                                               (dry basis                                         Ingredient       (wet basis)                                                                             and range)                                         ______________________________________                                        furfuryl alcohol 2.4       --                                                 polyamide imide solution                                                                       26.02     78.74                                              AI-10 from Amoco           (balance)                                          deionized water  66.48     --                                                 colloidal silica 4.60      15.86                                              Ludox AM from Du Pont      (10-20)                                            surfactant       0.5       5.4                                                Triton X100 from           (3-8)                                              Rohm & Haas Co.                                                               TOTAL            100.00    100.00                                             ______________________________________                                        PRIMER                                                                        Ingredient         % by weight (wet basis)                                    ______________________________________                                        furfuryl alcohol   1.85                                                       polyamide imide solution                                                                         18.09                                                      deionized water    52.01                                                      TiO.sub.2 -coated mica flake                                                                     0.06                                                       "Afflair" from Merck                                                          PTFE dispersion    20.95                                                      T-30 from Du Pont                                                             colloidal silica   3.62                                                       ultramarine blue pigment                                                                         3.15                                                       53% solids dispersed in water                                                 red vion oxide pigment                                                                           0.27                                                       45% solids dispersed in water                                                 TOTAL              100.00                                                     ______________________________________                                    

TOPCOAT

The preferred topcoat is that of example 1 of U.S. Pat. No. 4,118,537.

When the polysiloxane undercoat is used, no loss of adhesion is foundafter 8 hours immersion in boiling water. In comparison, the coating onPyrex glass starting with the second undercoat (PAI) without thepolysiloxane first undercoat completely delaminated from the substrateafter 5 minutes in boiling water.

Alternatively, the coats above the undercoat and primer (topcoat andoptional intermediate coats) can be those of U.S. Pat. Nos. 4,810,609;4,252,859; 4,351,882, and 4,425,448, and combinations thereof. Also thethin undercoats of the invention can also be used on substratesroughened in various ways known in the art to make coating systems evenbetter than without such undercoats.

I claim:
 1. A coated substrate comprising a substrate with multi-layer non-stick coating, comprising two undercoats, a primer, a topcoat, and up to one or more intermediate coats between the primer and the topcoat, wherein:the substrate is free of contaminants that would prevent adhesion of the undercoat, the first undercoat is applied to the substrate and consists essentially of polysiloxane resin and has a thickness in the range of 0.1-5.0 microns, the second undercoat is applied to the first undercoat and consists essentially of (on a weight basis after air drying but before baking) up to 8% surfactant, up to 20% fine-particle silica, the balance polyamic acid or polyamide imide, and has a dry film thickness in the range of 0.1-5.0 microns, the primer is applied to the second undercoat and comprises perfluorocarbon resin and at least one of polyamic acid or polyamide imide, polyarylene sulfide and polyether sulfone and has a dry film thickness in the range of 2-15 microns, and the topcoat is applied to the primer or the last of any intermediate coats and comprises perfluorocarbon resin, wherein the individual coating layers are partially dried between coatings, and the entire coating is cured together at temperatures above 416° C. after all the layers have been applied.
 2. The coated substrate of claim 1 wherein the coating comprises at least one intermediate coat containing the ingredients of the primer.
 3. The coated substrate of claim 1 wherein, before application of the undercoat, the surface of the substrate has been treated to remove contaminates that would interfere with adhesion but has not been etched or mechanically roughened.
 4. The coated substrate of claim 1 wherein the substrate is glass or ceramic.
 5. The coated substrate of claim 4 wherein the substrate before coating has a surface roughness profile less than 100 micro inches.
 6. The coated substrate of claim 4 wherein the substrate before coating has a surface roughness profile less than 50 micro inches.
 7. The coated substrate of claim 1 wherein the second undercoat contains 10-20% fine-particle silica and 3-8% surfactant.
 8. The coated substrate of claim 7 wherein the undercoat has a dry film thickness of 0.25-2.5 microns.
 9. The coated substrate of claim 8 wherein the primer has a dry film thickness of 5.0-7.5 microns. 